Hierarchy widget - preload children when requesting hierarchy widget for multiple work items

When we support fetching of multiple work items (#363299 (closed)), it would be good to make sure that we eliminate any N+1 issues when loading widget data. For hierarchy widget, we should be able to preload both parent and children for any issues being fetched.

---

Proposal

Preload required associations in WorkItemsResolver and include the children field in work_items_spec.rb.

Example query:

query {
  project(fullPath: "flightjs/flight") {
    workItems(types: ISSUE) {
      nodes {
        widgets {
        ... on WorkItemWidgetHierarchy {
              parent { id }
              children { nodes { id } }
            }
          }
        }
      }
    }
  }
}

N+1 queries:

 SELECT "issues".* FROM "issues"...
       -- (expected: 0, got: 1)
          INNER JOIN "work_item_parent_links" ON "issues"."id" = "work_item_parent_links"."work_item_id" WHERE "work_item_parent_links"."work_item_parent_id" = 91 ORDER BY "issues"."id" DESC LIMIT 101
       -- (expected: 0, got: 1)
          INNER JOIN "work_item_parent_links" ON "issues"."id" = "work_item_parent_links"."work_item_id" WHERE "work_item_parent_links"."work_item_parent_id" = 90 ORDER BY "issues"."id" DESC LIMIT 101
       -- (expected: 0, got: 1)
          INNER JOIN "work_item_parent_links" ON "issues"."id" = "work_item_parent_links"."work_item_id" WHERE "work_item_parent_links"."work_item_parent_id" = 89 ORDER BY "issues"."id" DESC LIMIT 101